A blast from the past – SPR-27 Vertical antenna for 27MHz

It is great to reminisce about the different types of base station antennas that were available for 27MHz Citizens band radios in the 1970’s and 1980’s.

In the 1970’s when 23 channel and later Australian 18 Channel equipment were available we will recall the Cushcraft “Ringo” half wave antenna, the Hustler ”Super Swamper” antenna which was 0.64 Wavelength and the legendary 5/8 Wave “Archer” ground plane antenna sold by Tandy Electronics.

In the early to mid 1980’s the American 40 Channel band plan was adopted and then we began to see a new breed of half wave base station antennas for 27 MHz. The average CBer would identify this new breed of antenna as being a “Stationmaster”. Back in the day, Stationmaster was the product name of a half wave antenna made by Antenna Achievements. There were other similar competing designs on the market, each with their own claim to fame.

These competing designs were the” V27” made by Mobile One Communications Systems in NSW and the “SPR-27” made by South Pacific Radio in Queensland. New, later day variants are still available, being the “Stationmaster MKI” sold by Shockwave Antennas and the “Grazy Master“, sold by G&C Communications Pty Ltd. The South Pacific Radio advertisement in CB Action magazine suggested that the design had been around almost forever but it wasn’t until December 1983 that we became aware of it.

These antennas were all a half wavelength tall with a large external tuning coil tapped off at the 50 ohm point. They generally covered from 26-29 MHz and were rated for 1KW – more than oodles for 4W of AM carrier and 12W Peak Envelope Power of Single Sideband. Electrically, the voltage at the base of a half wave antenna is a maximum and the impedance at this point may be 3000 or 4000 ohms. A matching circuit is required to allow us to connect our 50 ohm equipment to it. The nice thing about this design is that they are not cumbersome, are easy to erect and easy to tune – usually good to go straight out of the box.

It is amazing what a bucket of water and some wet and dry sandpaper can do to clean up such a relic. The usual electrolysis had occurred over the years to mounting clamp assemblies and where the tubes telescope into each other. The remainder of the aluminium tube sections were also oxidised. The orange conduit insulator has printed on it “Queensland made”.

Before mounting, the clamp assemblies and self tapping screws have been replaced. It is now time to clamp it to a mast and get out the Rig Expert AA-1400 antenna system analyser to see how it scrubs up for VSWR. It has cleaned up well for a thirty year old antenna. What is interesting is that the “ground” end of the matching coil terminates on the vertical part of the mounting bracket. On other designs like the V-27 and the Stationmaster the ground end attaches to the bottom part of the bracket near the SO239 connector.

The VSWR of this antenna is good and can be used for 27MHz Marine as is – if you are happy to operate at 1.7:1 VSWR without further adjustment. If it was desirable to peak its performance for 27MHz Marine then the top element would need to be lowered slightly and a new hole drilled for the self tapping screw. The tapping point on the coil may also need to be adjusted – all with the aid of a VSWR meter or antenna analyser of course.

In the next article we will look at a competing antenna – the Antenna Achievements Stationmaster MKI.

Andrew Pain